This invention relates to analog signal sensing circuits, and more particularly to analog sensor circuitry used in automotive ignition systems and the like. Even more particularly, this invention relates to an improved method for accurately detecting the threshold crossover events of an analog signal, and an improved crossover detector therefor having digital noise rejection accomplished without the use of hysteresis.
Threshold crossover detection of an analog signal is necessary for numerous applications. For example, crossover detectors are used in conjunction with a Hall sensor for the regulation of spark plug firing in conventional ignition systems. In these applications noise introduced through cross-talk with other circuits may interfere with the analog output of the Hall sensor, and may thus produce false threshold or zero crossover events. Such false events may initiate an untimed, erroneous spark plug response, thereby creating poor engine operation. To insure the accuracy of the detector, it is therefore desirable to remove the noise from the analog signal prior to the transmission of the signal to the spark plugs.
Many sensors operate at low analog input signal levels, and ideally at crossover produce a digital output signal. For sensors such as these, there is generally a trade-off between output signal accuracy (timing) and the ability to reject noise by means of hysteresis. Heretofore, efforts have been made to achieve noise rejection without sacrificing the accuracy of the output signal by selectively altering the effective hysteresis of a switching circuit during different portions of the analog input signal, as disclosed for example in my U.S. Pat. No. 3,916,328. This prior art, however, has revealed certain shortcomings of the analog method, such as the requirement of additional components for maintenance of hysteresis while switching analog levels.
Both U.S. Pat. Nos. 3,818,358 and 4,795,915 disclose what are termed as zero crossover digital noise rejection filters, but neither filter produces an output signal exactly at zero crossover of the input signal. More pertinent is U.S. Pat. No. 5,019,722, which discloses a threshold-crossing detection device with improved noise rejection that utilizes an integrated form of the sensor output to produce a binary signal having high and low levels. This binary signal is further processed for removal of multiple unwanted transitions resulting from noise. This prior art, however, has generally failed to produce a zero crossover detection device which is completely accurate and therefore free of output signal timing delays. Such timing delays are caused by the use of hysteresis or possibly other means for noise reduction.
It is therefore an object of this invention to provide an improved method of detecting a threshold crossover of the type described, and a novel detector whose digital output signal is accurately timed and substantially completely free of noise error.
Another object of this invention is to provide a threshold crossover detector which utilizes digital logic rather than hysteresis for rejection of noise present in an analog input signal, thus freeing the output signal from undesirable timing delays.
Other objects of this invention will become apparent hereinafter from the recital of the specification and appended claims, particularly when read in conjunction with the accompanying drawings.